This disclosure relates to an aircraft jet engine mounted fuel centrifugal boost pump, for example, in particular to the centrifugal boost pump volute.
The centrifugal boost pump is commonly packaged together with the main fuel pump, which is usually of a positive displacement gear pump type, both being driven by a common shaft. The fuel leaving the boost stage goes through a filter and a fuel oil heat exchanger before entering the main pump. Pressure losses are introduced by these components and the associated plumbing, while heat is also added to the fuel. The fuel feeding the centrifugal boost pump comes from the main frame fuel tanks through the main frame plumbing. The tanks are usually vented to the ambient atmospheric pressure, or, in some cases, are pressurized a couple of psi above that. The tanks are provided with immersed pumping devices, which are in some cases axial flow pumps driven by electric motors or turbines, or in other cases ejector pumps, collectively referred to as main frame boost pumps.
During flight, the pressure in the tank decreases with altitude following the natural depression in the ambient atmospheric pressure. Under normal operating conditions, industry standards require the main frame boost pumps to provide uninterrupted flow to the engine mounted boost pumps at a minimum of 5 psi above the true vapor pressure of the fuel and with no V/L (vapor liquid ratio) or no vapor present as a secondary phase. Under abnormal operation, which amounts to inoperable main frame boost pumps, the pressure at the inlet of the boost stage pumps can be only 2, or 3 psi above the fuel true vapor pressure, while vapor can be present up to a V/L ratio of 0.45, or more. Definition of terms, recommended testing practices, and fuel physical characteristics are outlined in industry specifications and standards like Coordinating Research Council Report 635, AIR 1326, (SAE Aerospace Information Report), SAE ARP 492 (SAE Aerospace Recommended Practices), SAE ARP 4024, (SAE Aerospace Recommended Practices), ASTM D 2779, (American Society for Testing and Materials), and ASTM D 3827 (American Society for Testing and Materials), for example.
During normal or abnormal operation, the centrifugal boost pump is required to maintain enough pressure at the main pump inlet under all the operating conditions encountered in a full flight mission such as the main pump can maintain the demanded output flow and pressure to the fuel control and metering unit for continuous and uninterrupted engine operation. There are also limitations in the maximum pressure rise the engine mounted centrifugal boost pump is allowed to deliver such not to exceed the mechanical pressure rating of the fuel oil heat exchanger, or limitations pertaining to minimum impeller blade spacing such as a large contaminant like a bolt lost from maintenance interventions would pass through and be trapped safely in the downstream filter. All these requirements along with satisfying a full flow operating range from large flows during takeoff to a trickle of flow during flight idle descent, and fuel temperature swings from −40 F to 300 F, makes the aerodynamic design of the engine mounted fuel pumps a serious challenge.
The volute collects the flow which is leaving the impeller in an almost tangential direction and with high velocities close to that of the impeller tip tangential velocity and directs it to the pump discharge port. From the pump inlet to the impeller exit port, the only element which adds power to the fluid is the impeller. The power is supplied at the shaft by the pump driver. A successful pump is expected to deliver the flow at the pump discharge port with relatively low velocities, at the required pressure rise above pump inlet pressure and with the best efficiency possible.
In general, impellers by themselves present high efficiencies between 75% and 95% depending on the pump size in terms of flow and running speed. The flow stream leaving the impeller exit port, aside from containing potential energy in the form of static pressure, also contains a fair amount of kinetic energy due to the high velocity of the fluid stream. Hence, in order to achieve a high overall efficiency for the entire pump, the volute must provide a high degree of pressure recovery, or transfer as much kinetic energy as possible into potential energy, or static pressure. To achieve this goal, the volute cross section is progressively increased in the direction of flow, which forces the fluid stream to slow down and, in the process, energy is recovered in the form of pressure.
The volute is composed of three distinct sections. The first section, which wraps around the impeller exit port, is called the volute proper. The second section, which usually is a straight tapered segment with a roundish cross section, is called a diffuser. The last section, which turns the flow from a normal plane relative to the impeller axis to an axial direction, is called exit bend. The need for the exit bend is dictated by the specific requirements of a given application.